How does quantum entanglement work? What does it
do? How could it be used or "generated" (produced)?

Question Date: 2007-09-18

Answer 1:

First, a little bit of background
information.

If we look at a car traveling
down the road, we know exactly where it is, and
how fast it's going, and it doesn't matter if we
are looking at the car or not, it just has a
certain position and speed. But in quantum
mechanics, there are properties of objects that
aren't well defined until we look at the object,
which is called taking a measurement.

For
example, let's say we draw an arrow on an
electron, which is one of the building blocks of
all the stuff around us. Let's say that arrow
could only be pointing up or down. According to
quantum mechanics, we can set it up so that the
arrow isn't actually pointing up or down until we
take a look to see which direction it's pointing.
Scientists say that the arrow is in a
superposition - it's pointing both up and down at
the same time. Let's say we measure the direction
of the arrow, and it's pointing up. After that,
it will definitely be pointing up. But before
that, it could have been pointing in either
direction. It took the act of measuring the
direction to actually force the arrow to be
pointing in one of the directions. It's kind of a
weird concept, but quantum mechanics has a lot of
weird concepts in it.

Quantum entanglement
is when we take two objects, and make it so that
they both have the same "quantum superposition".
It's similar to the example above, but now let's
say we have two electrons, and we've "entangled"
them somehow. Before we measure anything, the
arrows will still be in a superposition of up and
down - there's no way to say that it's pointing in
either direction. But once we measure the
direction of the arrow of one particle, we'll know
what the direction of the other particle's arrow
is. Since they were entangled, they'll have the
same properties, and knowing information about one
particle will tell us information about the other
particle, even if we haven't measured it
yet.

One way to generate entangled particles
would be by using something that is essentially
like a fancy prism. One important thing about
light is that scientists have found out that light
is actually made up of particles, called photons.
A photon is like the smallest amount of light we
could have. If we have a normal prism, we can
split regular sunlight into a rainbow. But if we
use a very specialized prism, and try to send one
photon through it, it's possible to get out two
photons that are entangled together.

One
application of quantum entanglement may be quantum
computing. A quantum computer would work kind of
like a normal computer, but it would use very,
very small objects, like electrons and photons, to
do calculations, instead of electricity.
Entangled particles would mean we could do the
same calculation more than once, which would help
check for any errors. Also, quantum entanglement
can be very useful for sending secret codes. We
could make a secret code using some of the
properties of electrons or photons. Then we could
create a new set of entangled electrons or photons
to send to someone else. The other person can be
sure that the information hasn't been looked at by
someone else if the particle is still in the
"superposition" state I mentioned above, since if
someone came and intercepted the message, that
measurement would ruin the entangled state.